1. Field
This invention relates generally to semiconductors, and more specifically, to external electrical connections to semiconductors.
2. Related Art
Integrated circuits are increasingly connected to each other by stacking two or more integrated circuits in a three dimensional (3D) stack. For example, this technique has been used to double an amount of memory by stacking two memory integrated circuits. An alternative form is to stack an integrated circuit with a semiconductor wafer. The stacked integrated circuits make electrical contact to each other through chip bonding. The chip bonding utilizes a bond pad on one integrate circuit that is bonded with another bond pad of the other integrated circuit. The bond pads are often referred to as “micropads”. Thermocompression bonding of micropads from two different die or wafers provide both mechanical and electrical inter-strata connections of the stacked die. A known bonding process uses a copper micropad at a bond pad of a first integrated circuit and copper and tin at a bond pad of a second integrated circuit. The two bond pads are aligned and boded together with the copper of the bond pad of the first integrated circuit bonding to the tin of the bond pad of the second integrated circuit. However, interdiffusion of the copper and tin of the bond pad of the second integrated circuit occurs at low temperature including room temperature to form intermetallic compounds such as Cu3Sn and Cu6Sn5. Thick intermetallic compounds are brittle and contribute to reliability issues associated with the bond pad. For example, a rigid pad made of an intermetallic compound is prone to shear and stress fractures. When a significant amount of intermetallic compound is formed, additional tin must be added to insure the material remains bondable. The additional tin increases the thickness which is undesired. Also, such intermetallic compounds are stable up to very high temperatures, such as six hundred degrees Celsius, and thus become non-bondable. Such high temperatures are too large to be useful for a bonding material because other portions of a completed integrated circuit are degraded or fail when subjected to such high temperatures.
Skilled artisans appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.